Physics Flashcards
Focal spot sizes
- mammo
- general X ray
- CT
- Mammo: 0.1 mm and 0.3 mm
- general X ray: 0.6 and 1.2 mm
- CT: 0.6 and 1.2 cm
Things that improve (decrease) heel effect
- focus to film distance: larger FFD = less heel effect
- film size: smaller film = less heel effect
- larger anode angle = less heel effect
Beam intensity
- what is it?
- what is it measured in?
Beam intensity = number of X rays x energy
- Roentgens per minute (R/min)
- increases in proportion to kvP ^ 2
five factors that change the x ray spectrum
- mAs: quantity
- kVp: quantity and quality
- voltage generator: quantity and quality
- filtration: decreases quantity, increases quality
- target material: changes characteristic peaks; higher atomic number increases X-ray quality and quality
Effect of mA and kVp on focal spot
- high mA, low kVp = wider focal spot (blooming)
- higher kVp = smaller spot (thinning)
Probability of PE
directly proportional to Z ^3
inversely proportional to incident photon energy ^ 3
Attenuation of X ray in tissue depends on what 3 things?
- Z
- tissue density
- X ray beam Quality
3 things that increase scatter
- higher KVP
- larger field of view
- thicker part (person)
limiting factor for spatial resolution
- for film
- for CR
- for DR
- flim: size of grain of photographic chemical
- CR: size of laser used to read the phosphor palte
- DR: size of individual thermoluminescent transistor
factors causing geometric unsharpness
- larger focal spot
- increased object to detector distance
- decreased source to object distance
- more magnification
Magnification factor
source to image distance / source to object distance
SID/SOD
modulation transfer function (image)
= information recorded/ information available
Susceptibility artifact
- worse with longer TE
- worse with GRE sequences
Adjusting mAs
- for fat people?
- for children 6-12
- for children 0-25
double mAs for every 4 cm of patient tissue
- 50% for age 6-12
- 25% for age 0-5
ways to reduce scatter
- collimate
- compression
- lower kVp
- grid or air gap
spatial resolution for
- film screen mammo?
- digital mammo?
- digital radiograph?
- CT
- MRI
- 15 lp/mm
- 7 lp/mm
- 3 lp/mm
- 0.7 lp/mm
- 0.3 lp/mm
Flat field test
Imaging large piece of acrylic; done to improve image quality and calibrate digital detectors in mammo
Target rate for MQSA audit
- recall rate
- cancers/1000 screened
5-7%
3-8 per 1000
MQSA processor QC
Daily
MQSA darkroom cleanliness
Daily
MQSA viewbox conditions
Weekly
MQSA phantom evaluation
Weekly
MQSA repeat analysis
Quarterly
MQSA compression test
Semi annually
MQSA darkroom fog
Semi annually
MQSA screen film contrast
Semi annually
MQSA line pair phantom
13 Lp/mm in anode cathode direction
11 lp/mm right to left
Limit on AERC exposure rate
87 mGy/ min or
10 R/min
How does electronic mag affect air Kerma and kerma area product?
Increases KERMA
Does not Change KAP
Air Kerma limit?
For fat people?
87 mGy/min (10 R/min)
High level control : 176 mGy/min (20 R/min)
Fill factor
Fill factor = sensitive area / pitch ^2
